hash/maphash: optimize Write and WriteString

The existing code makes copies of every byte it hashes.
When passed a large chunk of memory, Write and WriteString
can skip the copying and initSeed for most of it.

To ensure that Write, WriteByte, and WriteString continue to
generate output that depends only on the sequence of bytes,
expand the grouping test to include WriteString and interleaved calls.
Also, make the test process a lot more data, to ensure that
Write* handled full buffers correctly.

name            old time/op    new time/op    delta
Hash8Bytes-8      17.1ns ± 3%    16.5ns ± 2%   -3.26%  (p=0.000 n=29+27)
Hash320Bytes-8    74.9ns ± 2%    58.5ns ± 2%  -21.86%  (p=0.000 n=30+29)
Hash1K-8           246ns ± 3%     195ns ± 1%  -20.82%  (p=0.000 n=29+30)
Hash8K-8          1.87µs ± 2%    1.59µs ± 2%  -15.04%  (p=0.000 n=26+30)

name            old speed      new speed      delta
Hash8Bytes-8     468MB/s ± 3%   484MB/s ± 2%   +3.36%  (p=0.000 n=29+27)
Hash320Bytes-8  4.28GB/s ± 2%  5.47GB/s ± 2%  +27.97%  (p=0.000 n=30+29)
Hash1K-8        4.17GB/s ± 3%  5.26GB/s ± 1%  +26.28%  (p=0.000 n=29+30)
Hash8K-8        4.38GB/s ± 2%  5.16GB/s ± 2%  +17.70%  (p=0.000 n=26+30)

Updates #42710

Change-Id: If3cdec1580ffb3e36fab9865e5a9d089c0a34bec
Reviewed-on: https://go-review.googlesource.com/c/go/+/278758


Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>